It is known practice to manufacture molded parts using a gas counterpressure technique with an injection molding machine. An example of an injection molding method using gas counterpressure may be seen by referring to U.S. Pat. No. 5,662,841 issued to Milko G. Guergov. The Guergov method includes introducing molten plastic melt into a mold cavity against the force of pressurized gas in the mold cavity that counterbalances the internal pressure of the melt as it is injected into the cavity. This establishes a pressure balance for the plastic, the gas pressure being controlled within the cavity at a substantially constant value during the injection of the plastic into the cavity. A closed loop control uses pressure inputs from an internal melt pressure transducer so that the rate of feed of the plastic can be controlled by a hydraulic control system to reduce warp, to avoid external marks on the surface of the finished product, and to reduce cycle time.
The mold of Guergov is pressurized prior to injection. A logic controller for controlling the hydraulic system for a plastic melt feed nozzle maintains a constant pressure differential between the gas pressure in the cavity and the melt pressure throughout the injection process. As the melt solidifies in the mold, pressure adjustments are made using a closed loop control strategy. Each region of the molding solidifies with equal pressure. This balanced flow of the melt produces a finished part with reduced internal stress. The controller develops a closed loop PID signal in response to input information from an internal pressure transducer. The hydraulic unit uses the closed loop control signal to effect adjustments of the injection pressure, thereby maintaining continuous control over the internal melt pressure.